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                <h2 id="day4"><a href="#day4" class="headerlink" title="day4"></a>day4</h2><p>mybatis的缓存和注解开发<br>mybatis中的加载时机(查询的时机)<br>mybatis中的一级缓存和二级缓存<br>mybatis的注解开发<br>单表CRUD<br>多表查询</p>
<span id="more"></span>

<ol>
<li>mybatis延迟加载<br> 问题：在一对多中，当我们有一个用户，他有100个账户<br>   在查询用户的时候要不要把关联的账户查出来<br>   在查询账户的时候要不要把关联的用户查出来<br><br>   在查询用户时，用户下的账户信息应该是什么时候使用什么时候查询<br>   在查询账户时，账户的所属用户信息应该是随着账户查询时一起查询出来</li>
</ol>
<p>什么是延迟加载<br>    在真正使用数据时才发起查询，不用的时候不查询，按需加载<br>什么是立即加载<br>    不管用不用，只要一调用方法马上发起查询</p>
<p>在对应的四种表关系里<br>    一对多，多对多：通常情况下我们都是采用延迟加载</p>
<pre><code>    &lt;collection property=&quot;accounts&quot; ofType=&quot;account&quot; select=&quot;com.sqtian.dao.IAccountDao.findAccountByUid&quot; column=&quot;id&quot;&gt;&lt;/collection&gt;


配置findAccountByUid方法，findAll方法里只写select * from account，延迟开关打开时才会继续查询findAccountByUid
&lt;select id=&quot;findAccountByUid&quot; resultType=&quot;account&quot;&gt;
    select * from account where uid = #&#123;uid&#125;
&lt;/select&gt;

&lt;!--配置延迟开关的参数--&gt;
&lt;settings&gt;
    &lt;setting name=&quot;lazyLoadingEnabled&quot; value=&quot;true&quot;/&gt;
    &lt;setting name=&quot;aggressiveLazyLoading&quot; value=&quot;false&quot;/&gt;
&lt;/settings&gt;

多对一，一对一：通常情况下我们都是采用立即加载
</code></pre>
<ol start="2">
<li>Mybatis中的缓存<br> 什么是缓存：存在于内存中的临时数据<br> 为什么使用缓存：减少与数据库的交互次数，提高执行效率<br> 适用于缓存：<br> 经常查询并且不经常改变<br> 数据正确与否对最终结果影响不大<br> 不使用于缓存：<br> 经常改变的数据<br> 数据得正确与否对最终结果影响很大<br> 例如：商品的库存，银行的汇率，股市的牌价<br> 一级缓存<br> 它指的是Mybatis中SqlSession对象的缓存<br> 当我们执行查询之后，查询的结果会同时存入到SqlSession为我们提供一块区域中<br> 该区域的结构是一个Map。当我们再次查询同样的数据，mybatis会先去sqlsession中查询是否有，有的话直接拿出来用<br> 当SqlSession对象消失时，mybatis的一级缓存也就消失了<br> 二级缓存<br> 它指的是Mybatis中SqlSessionFactory对象的缓存，由同一个SqlSessionFactory对象创建的SqlSession共享其缓存<br><br> 使用二级缓存：<br>     第一步：让Mybatis框架支持二级缓存（在SqlMapConfig.xml中配置）<setting name = "cacheEnabled" value="true"/><br>     第二步: 让当前的映射文件支持二级缓存(在IUserDao.xml中配置)<cache/><br>     第三步: 让当前的操作支持二级缓存(在select标签中配置)<select useCache = "true"/><br><br>     存放在二级缓存的内容是数据,而不是对象.谁要用,就把数据拿来新建一个用户对象</li>
</ol>
<h2 id="Mybatis注解开发"><a href="#Mybatis注解开发" class="headerlink" title="Mybatis注解开发"></a>Mybatis注解开发</h2><p>注解与IUserDao.xml不能同时使用<br>jdbcConfig.properties</p>
<h2 id="jdbc-driver-x3D-com-mysql-jdbc-Driverjdbc-url-x3D-jdbc-mysql-x2F-x2F-localhost-3306-x2F-lovelycc-serverTimezone-x3D-UTC-amp-characterEncoding-x3D-utf8jdbc-username-x3D-rootjdbc-password-x3D-root"><a href="#jdbc-driver-x3D-com-mysql-jdbc-Driverjdbc-url-x3D-jdbc-mysql-x2F-x2F-localhost-3306-x2F-lovelycc-serverTimezone-x3D-UTC-amp-characterEncoding-x3D-utf8jdbc-username-x3D-rootjdbc-password-x3D-root" class="headerlink" title="jdbc.driver&#x3D;com.mysql.jdbc.Driverjdbc.url&#x3D;jdbc:mysql:&#x2F;&#x2F;localhost:3306&#x2F;lovelycc?serverTimezone&#x3D;UTC&amp;characterEncoding&#x3D;utf8jdbc.username&#x3D;rootjdbc.password&#x3D;root"></a>jdbc.driver&#x3D;com.mysql.jdbc.Driver<br>jdbc.url&#x3D;jdbc:mysql:&#x2F;&#x2F;localhost:3306&#x2F;lovelycc?serverTimezone&#x3D;UTC&amp;characterEncoding&#x3D;utf8<br>jdbc.username&#x3D;root<br>jdbc.password&#x3D;root</h2><p>CRUD注解：不用再去xml里操作</p>
<p>&#x2F;**</p>
<ul>
<li><p>在mybatis中CRUD一共有四个注解</p>
</li>
<li><p>@Select @Insert @Update @Delete<br> *&#x2F;<br>public interface IUserDao {</p>
<p> &#x2F;**</p>
<ul>
<li>查询所有用户</li>
<li>@return<br>  *&#x2F;<br> @Select(“select * from hero”)<br> List<User> findAll();</li>
</ul>
<p> &#x2F;**</p>
<ul>
<li>保存用户<br>  *&#x2F;<br> @Insert(“insert into hero values(#{id},#{name},#{age},#{weight},#{height})”)<br> void saveUser(User user);</li>
</ul>
<p> &#x2F;**</p>
<ul>
<li>更新用户<br>  *&#x2F;<br> @Update(“update hero set name&#x3D;#{name},age&#x3D;#{age},weight&#x3D;#{weight},height&#x3D;#{height} where id&#x3D;#{id}”)<br> void updateUser(User user);</li>
</ul>
<p> &#x2F;**</p>
<ul>
<li>删除用户<br>  *&#x2F;<br> @Delete(“delete from hero where id&#x3D;#{id}”)<br> void deleteUser(Integer uid);</li>
</ul>
</li>
</ul>
<h2 id=""><a href="#" class="headerlink" title="}"></a>}</h2><p>public class AnnotationCRUDTest {</p>
<pre><code>private InputStream in;
private SqlSession sqlSession;
private IUserDao userDao;

/**
 * 测试查询所有
 */
@Before //用于在测试方法执行之前执行
public void init() throws Exception&#123;
    //1.读取配置文件,生成字节输入流
    in = Resources.getResourceAsStream(&quot;SqlMapConfig.xml&quot;);
    //2.获取SqlSessionFactory
    SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
    //3.获取sqlSession对象
    sqlSession = factory.openSession();
    //4.获取dao代理对象
    userDao = sqlSession.getMapper(IUserDao.class);
&#125;

@After //用于在测试方法执行之后执行
public void destroy() throws IOException &#123;
    //提交事务
    sqlSession.commit();
    //6.关闭资源
    sqlSession.close();
    in.close();

&#125;

@Test
public void testSave()&#123;
    User user = new User();
    user.setId(14);
    user.setName(&quot;xx&quot;);
    user.setAge(18);
    user.setWeight(90);
    user.setHeight(165);
    userDao.saveUser(user);

&#125;

@Test
public void testUpdate()&#123;
    User user = new User();
    user.setId(14);
    user.setName(&quot;叶雨时&quot;);
    user.setAge(21);
    user.setWeight(126);
    user.setHeight(180);
    userDao.updateUser(user);

&#125;

@Test
public void testDelete()&#123;
    userDao.deleteUser(14);
    List&lt;User&gt; users = userDao.findAll();
    for (User u : users) &#123;
        System.out.println(u);
    &#125;
&#125;
</code></pre>
<p>}</p>
<hr>
<p>result注解：</p>
<pre><code>@Select(&quot;select * from hero&quot;)
@Results(id = &quot;userMap&quot;,value=&#123;
    @Result(id=true,column = &quot;id&quot;,property = &quot;id&quot;),
    @Result(id=false,column = &quot;name&quot;,property = &quot;name&quot;),
    @Result(id=false,column = &quot;age&quot;,property = &quot;age&quot;),
    @Result(id=false,column = &quot;weight&quot;,property = &quot;weight&quot;),
    @Result(id=false,column = &quot;height&quot;,property = &quot;height&quot;)
&#125;)
List&lt;User&gt; findAll();


@Insert(&quot;insert into hero values(#&#123;id&#125;,#&#123;name&#125;,#&#123;age&#125;,#&#123;weight&#125;,#&#123;height&#125;)&quot;)
@ResultMap(&quot;userMap&quot;)
void saveUser(User user);
</code></pre>
<p>一对一：一个账户只有一个用户<br>    1. 在Account中添加User属性<br>    2. 在注解中添加映射<br>    3. 用User的FindById找到User</p>
<hr>
<pre><code>//多对一(mybatis中称之为一对一)的映射: 一个账户只能属于一个用户
private User user;

public User getUser() &#123;
    return user;
&#125;

public void setUser(User user) &#123;
    this.user = user;
&#125;
</code></pre>
<hr>
<pre><code>@Select(&quot;select * from hero where id=#&#123;id&#125;&quot;)
@ResultMap(&quot;userMap&quot;)
User findById(Integer uid);
</code></pre>
<hr>
<p>public interface IAccountDao  {</p>
<pre><code>/**
 * 查询所有账户,并且获取每个账户所属的用户信息
 * 1对1,使用立即加载
 */

@Select(&quot;select * from account&quot;)
@Results(id = &quot;accountMap&quot;,value = &#123;
    @Result(id=true,column = &quot;id&quot;,property = &quot;id&quot;),
    @Result(column = &quot;uid&quot;,property = &quot;uid&quot;),
    @Result(column = &quot;name&quot;,property = &quot;name&quot;),
    @Result(column = &quot;balance&quot;,property = &quot;balance&quot;),
    @Result(column = &quot;uid&quot;,property = &quot;user&quot;,one=@One(select=&quot;com.sqtian.dao.IUserDao.findById&quot;,fetchType = FetchType.EAGER))
&#125;)
List&lt;Account&gt; findAll();
</code></pre>
<p>}</p>
<hr>
<pre><code>@Test
public void testFindAll()&#123;
    List&lt;Account&gt; accounts = accountDao.findAll();
    for (Account account : accounts) &#123;
        System.out.println(&quot;---每一个账户的信息---&quot;);
        System.out.println(account);
        System.out.println(account.getUser());
    &#125;
&#125;
</code></pre>
<hr>
<p>一对多：一个用户可以有多个账户<br>    1. 在用户里添加accounts属性<br>    2. IUserDao的注解里添加映射<br>    3. 利用account的方法findByUid</p>
<pre><code>//1对多关系映射,一个用户对应多个账户
private List&lt;Account&gt; accounts;

public List&lt;Account&gt; getAccounts() &#123;
    return accounts;
&#125;

public void setAccounts(List&lt;Account&gt; accounts) &#123;
    this.accounts = accounts;
&#125;
</code></pre>
<hr>
<pre><code>/**
 * 查询所有用户
 * 一对多,使用延迟加载
 * @return
 */
@Select(&quot;select * from hero&quot;)
@Results(id = &quot;userMap&quot;,value=&#123;
    @Result(id=true,column = &quot;id&quot;,property = &quot;id&quot;),
    @Result(id=false,column = &quot;name&quot;,property = &quot;name&quot;),
    @Result(id=false,column = &quot;age&quot;,property = &quot;age&quot;),
    @Result(id=false,column = &quot;weight&quot;,property = &quot;weight&quot;),
    @Result(id=false,column = &quot;height&quot;,property = &quot;height&quot;),
    @Result(property = &quot;accounts&quot;,column = &quot;id&quot;,
        many=@Many(select = &quot;com.sqtian.dao.IAccountDao.findByUid&quot;,fetchType = FetchType.LAZY))

&#125;)
List&lt;User&gt; findAll();
</code></pre>
<hr>
<pre><code>/**
 * 根据用户id查询账户信息
 * @param uid 用户id
 * @return
 */
@Select(&quot;select * from account where uid=#&#123;id&#125;&quot;)
List&lt;Account&gt; findByUid(Integer uid);
</code></pre>
<hr>
<pre><code>@Test
public void testFindAll()&#123;
    List&lt;User&gt; users = userDao.findAll();
    for (User user : users) &#123;
        System.out.println(&quot;---每一个用户信息---&quot;);
        System.out.println(user);
        System.out.println(user.getAccounts());
    &#125;

&#125;
</code></pre>
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